Introduction to Computing II Lecture 6: Lists, Stacks, and Queues

308-203AIntroduction to Computing IILecture 6: Lists, Stacks, and Queues Fall Session 2000

Value a b c d Next To glue data together... Node = List = a chain of nodes

a b c d Terminology “head” “links” “tail”

Possible Operations on Lists • Locate an element • Check if empty • Check the number of items in the list • Concatenate lists together • Insert an element (where?) • Delete an element (how is it referred to?)

A Simple Java Implementation Class List { Node head; // methods …. } Class Node { Object value; Node next; // methods …. }

Orders of Growth Q1. What is the complexity of adding an item at the head of a list? Q2. What is the complexity of adding an item at the tail of a list? Q3. In general?

Orders of Growth Q1. What is the complexity of adding an item at the head of a list? A1. O(1) Q2. What is the complexity of adding an item at the tail of a list? A2. O(n) Q3. In general? A3. O(n)

a b c d A Variation: Doubly Linked Lists Value Next Node = Previous

Orders of Growth Q1. What is the complexity of adding an item at the head of a doubly-linked list? Q2. What is the complexity of adding an item at the tail of a doubly-linked list? Q3. In general?

Orders of Growth Q1. What is the complexity of adding an item at the head of a doubly-linked list? A1. O(1) Q2. What is the complexity of adding an item at the tail of a doubly-linked list? A2. O(1) Q3. In general? A3. O(n)

Stacks Definition: A stack is a list which is accessed only by the two following methods: 1. Push - insert an element at the head 2. Pop - remove the most recently “pushed” element

Stacks - the intuition Think of pancakes on a plate c b b b a a a a a Push(a) Empty Push(b) Push(c) Pop( ) Pop( ) c b

Why do we care? A stack has less functionality than a list (because we only pop and push) but it’s enough to do some important things: 1. Reverse Polish Notation calculators 2. This is really what happens when you call a function or method 3. CS Profs like to ask questions about stacks on exams.

Queues(aka FIFO) Definition: A queue is a list which is accessed only by the two following methods, which preserve a First-In First-Out or “FIFO” order: 1. Insert - insert an element at one end 2. Dequeue - remove the “oldest” element (= the one at the other end of the list)

Queues The intuition: this is what you do when you’re waiting in line at the grocery store Queue a d c b Cashiers service customers Customers in ( = INSERT) ( = DEQUEUE)

Why do we care? Whenever you’ve got a lot of tasks to do, putting them in a queue and servicing them in FIFO order guarantees that no task goes forever unfinished…. True for HTTP requests on a webserver as for the client in line at the bank.

Orders of growth?? Q1. What is the complexity of the two queue operations if there is an underlying (singly-linked) list? Q2. If it’s a doubly-linked list??

Orders of growth?? Q1. What is the complexity of the two queue operations if there is an underlying (singly-linked) list? One is O(n), the other O(1) Q2. If it’s a doubly-linked list?? Both O(1)

A Class Hierarchy ofList-Like Objects SimpleList List Stack Queue This lives on our website under “Examples”

Any questions?

Introduction to Computing II Lecture 6: Lists, Stacks, and Queues